In an ion trap device, ions are trapped by a three-dimensional quadrupole electric field formed by a combination of an RF (radio frequency) electric field and a DC (direct current) electric field. There are some types of ion trap device, including one using electrodes having hyperboloid-of-revolution inner surfaces, and another using a cylindrical electrode and a pair of circular plate electrodes placed at both ends of the cylindrical electrodes. The former one having the hyperboloid-of-revolution inner surfaces can form a larger ion trapping region in the space surrounded by the electrodes, and the latter one using the cylindrical and circular plate electrodes has rather narrower ion trapping region. In any type of the ion trap device, the electrode surrounding circularly the ion trapping space is called a ring electrode, and the electrodes placed at both ends of the ring electrode are called end cap electrodes. Normally, the RF voltage is applied to the ring electrode to form the trapping electric field. In any type of the ion trap device, the mass to charge ratio of an ion determines whether it is securely and stably trapped in the ion trapping space, or it moves irregularly and collides with an inner surface of the electrodes or is ejected outside through an opening of the electrodes. The kinetics of the ions in the ion trapping space is described in detail in, for example, R. E. March and R. J. Hughes, “Quadrupole Storage Mass Spectrometry”, John Wiley & Sons, 1989, pp. 31-110.
In a typical structure for applying an RF voltage to the ring electrode, a coil is connected to the ring electrode, where the inductance of the coil, the capacitance between the ring electrode and the pair of end cap electrodes and the capacitance of all the other elements constitute an LC resonant circuit. To the LC resonant circuit, an RF driver (or an RF exciting circuit) is connected directly or indirectly through a transformer coupling. In such a structure, a large amplitude RF voltage (RF high voltage) can be applied to the ring electrode with a small amplitude driving voltage owing to the high Q value of the LC resonant circuit. In order to enhance the amplifying efficiency of the LC resonant circuit, a tuning circuit including a variable capacitor is normally used to make the resonance frequency of the LC circuit coincide with the frequency of the RF driver.
When the temperature rises, the coil may swell and its inductance may change, or the capacitance of the variable capacitor may change. This causes the resonance frequency of the resonant circuit to shift from that of the RF driver. In one case, a high voltage switch is connected to the ring electrode. When the RF high voltage is changed, the capacitance of the high voltage switch may change and the resonance may break. Generally a feedback control is incorporated to fix the amplitude of the RF high voltage to a target value by adjusting the output voltage of the RF driver, so that the amplitude of the RF high voltage is stable irrespective of the shift of the resonance frequency.
But there arises an error, or a shift, in a relative phase between the output of the RF driver and the amplified RF high voltage. When some ion processing is made in the ion trap device using, or relating to, the phase of the RF high voltage, such as an ion selection processing or an ion dissociation processing, the phase of the RF voltage is deduced from the phase of the RF driver, and various timings are determined based on the phase thus determined. Thus, when there arises a shift in the relative phase between the output of the RF driver and the RF high voltage, the processing cannot be done properly or the precision of the processing deteriorates.
When, for example, an ion mass analysis is made by changing, or scanning, the amplitude of the RF high voltage, the timing when the ions are ejected from the ion trapping space is related to the phase of the RF high voltage. If there is a shift in the phase, the position of a peak or peaks of the mass spectrum shifts accordingly. When, for example, ions are extracted from an ion trap device to a TOF mass spectrometer, the position of a peak or peaks of the mass spectrum also shifts if there is a shift in the phase of the RF high voltage because ion's energy and direction of motion at a timing of extraction is closely related to the phase.
Such a problem can be solved, in principle, by monitoring (not the output of the RF driver but) the RF high voltage which is generated through amplification by resonance, detecting the phase of the RF high voltage directly, and then using the detected phase as the basis of the control. But, actually, it is very difficult to always detect an exact phase of the RF high voltage which alters in many ways. Even if it is possible in any way, it is too expensive to be practical. Another problem is that installing such a function to an existing mass spectrometer is practically impossible.
The present invention addresses the problem, and an object of the invention is to decrease the shift in the phase difference between the output of the RF driver and the RF high voltage. This will alleviate or prevent deterioration of the mass analysis or other processings using the ion trap device caused by the shift in the phase of the RF high voltage.
Thus an ion trap device according to the present invention includes:
a ring electrode and a pair of end cap electrodes;
an RF driver for generating a driving voltage with a driving frequency;
a resonant circuit for amplifying the driving voltage generated by the RF driver to produce an RF voltage applied to at least one of the electrodes; and
a tuning circuit for changing a resonance frequency of the resonant circuit, wherein the tuning circuit is adjusted so that the resonance frequency is shifted from the driving frequency.
According to the present invention, in a method of tuning an ion trap device which includes
a ring electrode and a pair of end cap electrodes,
an RF driver for generating a driving voltage with a driving frequency,
a resonant circuit for amplifying the driving voltage generated by the RF driver to produce an RF voltage applied to at least one of the electrodes, and
a tuning circuit for changing a resonance frequency of the resonant circuit, the tuning circuit is adjusted so that the resonance frequency of the resonant circuit is shifted from the driving frequency.
The principle of the present invention is explained using FIG. 2, which shows a model diagram of an LCR series-resonance circuit. In the circuit, the capacitor 101 representative of the overall capacitance of the circuit including the capacitance formed between the electrodes is C. The inductance of the coil 102 is L, and the effective resistance 103 of the resonant circuit is R. The angular frequency of the driving voltage (output) of the RF driver 100 is ω, and the angular resonance frequency of the resonant circuit is ω0. The impedance Z of the resonant circuit is:Z=R+jXwhere X=ωL−1/(ωC). When ω=ω0, the resonance condition is satisfied, and X=0. At this condition, the impedance Z reaches its minimum value of R. This means that the objective RF high voltage is obtained with a minimum driving voltage through amplification. The gain of the amplification is called the Q-value, which is given by Q=ω0L/R.In many ion trap devices, the Q-value of the resonant circuit is set at around 100-300.
When the driving voltage is V0, the current I flowing through the resonant circuit is represented byI=V0/Z.The RF high voltage VRF generated between the electrodes of the ion trap device corresponds to the voltage across the capacitor C in the model circuit. Since the impedance of the capacitor C is represented by−j/(ωC)≈−jω0L,the RF high voltage VRF is represented byVRF=(−jω0L/Z)·V0.Thus the phase difference θ between the output of the RF driver 100 and the RF high voltage is given byθ=−π/2−∠Z,where ∠Z is the angle of the impedance Z. By rewriting the reactance X toX=QR·(ω/ω0−ω0/ω),the angle of Z is given bytan(∠Z)=Q·(ω/ω0−ω0/ω).Differentiating both sides of the above equation by the angular frequency, the shift Δθ in the phase difference θ is given byΔθ≈2Q·cos2(∠Z)·(Δω/ω0)This means that the phase shift Δθ, caused by a fixed amount of the shift in the angular frequency Δω, can be decreased by shift of resonance frequency from the resonance condition ∠Z=0. For example, the phase shift Δθ is about 0.25 times (a quarter) at ∠Z=60° compared to that in the resonance condition, i.e., ∠Z=0. The ratio decreases according to cos2(∠Z): for example, when ∠Z=65°, the ratio is 0.179, and when ∠Z=70°, the ratio is 0.117.
Thus, in the present invention, the resonance frequency of the resonant circuit, which is used to apply the RF high voltage to one of the electrodes of the ion trap device, is deliberately shifted from the frequency of the RF driver (driving frequency). This reduces the influence of the deviation in the resonance frequency caused by the change in the RF high voltage on the shift in the phase difference between the output of the RF driver and the RF high voltage. This minimizes the degradation of various performances of the ion trap device relating to the phase difference, such as the shift in the peaks of the mass spectrum and enhances the sensitivity and precision of the mass analysis of the mass spectrometers using the ion trap device.
If the phase difference between the output of the RF driver and the RF high voltage depends on the amplitude of the RF voltage, the resonant circuit may not be stable unless the shift of the resonance frequency from the resonance condition is made in a proper direction. For example, when a semiconductor device is connected to the electrode (or electrodes) to which the RF voltage is applied, the effective capacitance of the semiconductor device increases as the RF voltage increases, which leads to the decrease in the resonance frequency of the resonant circuit. Suppose that, in such a case, the resonance frequency of the resonant circuit is shifted in the direction of increasing frequency by decreasing the capacitance of the resonant circuit. If the amplitude of the RF high voltage is increased, the capacitance increases, which brings the resonant circuit toward the resonance condition. This increases the gain of the resonant circuit, and destabilize the resonance due to the positive feedback phenomenon.
Thus, when a semiconductor device is connected to the electrode (or electrodes) to which the RF high voltage is applied as described above, it is recommended to shift the resonance frequency in the direction of decreasing frequency by, for example, increasing the capacitance using a variable capacitor, which functions as the tuning circuit mentioned above. Generally speaking, when the resonance frequency shifts in a direction toward another frequency as the RF voltage increases, the tuning procedure should shift the resonance frequency of the resonant circuit in the same direction. This stabilizes the resonance.